The desirable properties of aluminum and its alloys such as low cost, low density, corrosion resistance, and ease of fabrication are well known.
An important means for enhancing the strength of some aluminum alloys is heat treatment. Conventionally, three basic steps are employed in the heat treatment of aluminum alloys: (1) Solution heat treating; (2) Quenching; and (3) Aging. Additionally, a cold working step is often added prior to aging. Solution heat treating consists of soaking the alloy at a temperature sufficiently high and for a long enough time to achieve a nearly homogeneous solid solution of precipitate-forming elements in aluminum. The objective is to take into solid solution the maximum practical amounts of the soluble hardening elements. Quenching involves the rapid cooling of the solid solution, formed during the solution heat treatment, to produce a supersaturated solid solution at room temperature. The aging step involves the formation of strengthening precipitates from the rapidly cooled supersaturated solid solution. Precipitates may be formed using natural (ambient temperature), or artificial (elevated temperature) aging techniques. In natural aging, the quenched alloy is held at temperatures in the range of -20.degree. to +50.degree. C., typically at room temperature, for relatively long periods of time. For certain alloy compositions, the precipitation hardening that results from natural aging alone produces useful physical and mechanical properties. In artificial aging, the quenched alloy is held at temperatures typically in the range of 100.degree. to 200.degree. C. for periods of approximately 5 to 48 hours, typically, to effect precipitation hardening.
The extent to which the strength of Al alloys can be increased by heat treatment is related to the type and amount of alloying additions used. The addition of copper to aluminum alloys, up to a certain point, improves strength, and in some instances enhances weldability. The further addition of magnesium to Al-Cu alloys can improve resistance to corrosion, enhance natural aging response without prior cold work and increase strength. However, at relatively low Mg levels, i.e., 1.5 percent, weldability is decreased.
One commercially available aluminum alloy containing both copper and magnesium is Aluminum Association registered alloy 2024, having nominal composition Al--4.4 Cu--1.5 Mg--0.6 Mn. Alloy 2024 is a widely used alloy with high strength, good toughness, good warm temperature properties and a good natural-aging response. However, its corrosion resistance is limited in some tempers, it does not provide the ultra high strength and exceptionally strong natural-aging response achievable with the alloys of the present invention, and it is only marginally weldable. In fact, 2024 welded joints are not considered commercially useful in most situations.
Another commercial Al-Cu-Mg alloy is Aluminum Association registered alloy 2519 having a nominal composition of Al--5.6 Cu--0.2 Mg--0.3 Mn--0.2 Zr--0.06 Ti--0.05 V. This alloy was developed by Alcoa as an improvement on alloy 2219, which is presently used in various aerospace applications. While the addition of Mg to the Al-Cu system can enable a natural-aging response without prior cold work, alloy 2519 has only marginally improved strengths over alloy 2219 in the highest strength tempers.
Work reviewed by Mondolfo on conventional Al-Cu-Mg alloys indicates that the main hardening agents are CuAl.sub.2 type precipitates in alloys in which the Cu to Mg ratio is greater than 8 (See ALUMINUM ALLOYS: STRUCTURE AND PROPERTIES, L. F. Mondolfo, Boston: Butterworths, 1976, p. 502).
Polmear, in U.S. Pat. No. 4,772,342, has added silver and magnesium to the Al-Cu system in order to increase elevated temperature properties. A preferred alloy has the composition Al--6.0 Cu--0.5 Mg--0.4 Ag--0.5 Mn--0.15 Zr--0.10 V--0.05 Si. Polmear associates the observed increase in strength with the "omega phase" that arises in the presence of Mg and Ag (see "Development of an Experimental Wrought Aluminum Alloy for Use at Elevated Temperatures," Polmear, ALUMINUM ALLOYS: THEIR PHYSICAL AND MECHANICAL PROPERTIES, E. A. Starke, Jr. and T. H. Sanders, Jr., editors, Volume I of Conference Proceedings of International Conference, University of Virginia, Charlottesville, Va., 15-20 Jun. 1986, pages 661-674, Chameleon Press, London).
Adding lithium to Al-Mg alloys and to Al-Cu alloys is known to lower the density and increase the elastic modulus, producing significant improvements in specific stiffness and enhancing the artificial age hardening response. However, conventional Al-Li alloys generally possess relatively low ductility at given strength levels and toughness is often lower than desired, thereby limiting their use.
Difficulties in melting and casting have limited the acceptance of Al-Li alloys. For example, because Li is extremely reactive, Al-Li melts can react with the refractory materials in furnace linings. Also, the atmosphere above the melt should be controlled to reduce oxidation problems. In addition, lithium lowers the thermal conductivity of aluminum, making it more difficult to remove heat from an ingot during direct-chill casting, thereby decreasing casting rates. Furthermore, in Al-Li melts containing 2.2 to 2.7 percent lithium, typical of recently commercialized Al-Li alloys, there is considerable risk of explosion. To date, the property benefits attributable to these new Al-Li alloys have not been sufficient to offset the increase in processing costs caused by the above-mentioned problems. As a consequence they have not been able to replace conventional alloys such as 2024 and 7075. The preferred alloys of the present invention do not create these melting and casting problems to as great a degree because of their lower Li content.
Al-Li alloys containing Mg are well known, but they typically suffer from low ductility and low toughness. One such system is the low density, weldable Soviet alloy 01420 as disclosed in British Patent 1,172,736, to Fridlyander et al, of nominal composition Al--5 Mg--2 Li. This alloy is reported to have medium to high strength, low density, and a modulus of elasticity higher than standard aluminum alloys.
A paper appearing in the Journal of Japan Institute of Light Metals lists Al-Li-Mg base alloys to which minor amounts of one of the elements Ag, Cu, or Zn has been added (see "Aging Phenomena of Al-Li-Mg Alloy Affected by Additional Elements," Hayashi et al, Journal of Japan Institute of Light Metals, Vol. 32, No. 7, July 1982). The authors studied the effect of each individual alloying element on the aging behavior of ternary Al-Li-Mg alloys. The authors did not combine Ag, Cu or Zn alloying additions, nor did they add grain refining elements to their alloys.
U.S. Pat. No. 3,346,370 to Jagaciak et al discloses Al-Mg base alloys to which minor amounts of Li in the range of 0.01-0.8 percent may be added. The alloys may also contain up to 0.72 percent Cu and up to 0.35 percent Zn.
Al-Li alloys containing Cu are also well known, such as alloy 2020, which was developed in the 1950's, but was withdrawn from production because of processing difficulties, low ductility and low fracture toughness. Alloy 2020 falls within the range disclosed in U.S. Pat. No. 2,381,219 to LeBaron, which teaches that the alloys are "magnesium-free", i.e., the alloys have less than 0.01 percent Mg, which is present only as an impurity. The alloys disclosed by LeBaron also require the presence of at least one element selected from Cd, Hg, Ag, Sn, In and Zn. The reference teaches that when zinc is used, levels below 0.5 percent are employed, with levels between 0.01 and 0.05 percent being preferred, due to the tendency for zinc to increase brittleness at higher levels.
To achieve the highest strengths in Al-Cu-Li alloys, it is necessary to introduce a cold working step prior to aging, typically involving stretching and/or rolling of the material at ambient or near ambient temperatures. The strain which is introduced as a result of cold working produces dislocations within the alloy which serve as nucleation sites for the strengthening precipitates. In particular, conventional Al-Cu-Li alloys must be cold worked before artificial aging in order to obtain high strengths, i.e., greater than 70 ksi ultimate tensile strength (UTS). Cold working of these alloys is necessary to promote high volume fractions of Al.sub.2 CuLi (T.sub.1) and Al.sub.2 Cu (theta-prime) precipitates which, due to their high surface-to-volume ratio, nucleate far more readily on dislocations than in the aluminum solid solution matrix. Without the cold working step, the formation of the plate-like Al.sub.2 CuLi and Al.sub.2 Cu precipitates is retarded, resulting in significantly lower strengths. Moreover, the precipitates do not easily nucleate homogeneously because of the large energy barrier that has to be overcome due to their large surface area. Cold working is also useful, for the same reasons, to produce the highest strengths in many commercial Al-Cu alloys, such as 2219.
The requirement for cold working to produce the highest strengths in Al-Cu-Li alloys is particularly limiting in forgings, where it is often difficult to uniformly introduce cold work to the forged part after solutionizing and quenching. As a result, forged Al-Cu-Li alloys are typically limited to non-cold worked tempers, resulting in generally unsatisfactory mechanical properties.
Recently, Al-Li alloys containing both Cu and Mg have been commercialized. These include alloys 8090, 2091, 2090, and CP 276. Alloy 8090, which is described in U.S. Pat. No. 4,588,553 to Evans et al, contains 1.0-1.6 Cu, 2.2-2.7 Li, and 0.6-1.3 Mg. The alloy was designed with the following properties for aircraft applications: good exfoliation corrosion resistance, good damage tolerance, and a mechanical strength greater than or equal to 2024 in T3 and T4 conditions. Alloy 8090 does exhibit a natural-aging response without prior cold work, but not nearly as strong as that of the alloys of the present invention. In addition, 8090-T6 forgings have been found to possess low transverse elongation.
Alloy 2091, with 1.8-2.5 Cu, 1.7-2.3 Li, and 1.1-1.9 Mg, was designed as a high strength, high ductility alloy. However, at heat treated conditions that produce maximum strength, ductility is relatively low in the short transverse direction. Additionally, strengths achieved by alloy 2091 in non-cold worked tempers are significantly below those attained by the alloy in cold-worked tempers.
In recent work on alloys 8090 and 2091, Marchive and Charue have reported reasonably high longitudinal tensile strengths (see "Processing and Properties", 4TH INTERNATIONAL ALUMINUM LITHIUM CONFERENCE, G. Champier, B. Dubost, D. Miannay, and L. Sabetay editors, Proceedings of International Conference, 10-12 Jun. 1987, Paris, France, pp. 43-49). In the T6 temper, 8090 possesses a yield strength of 67.3 ksi and an ultimate tensile strength of 74 ksi, while 2091 possesses a yield strength of 63.8 ksi and an ultimate tensile strength of 75.4 ksi. However, the strengths of both 8090-T6 and 2091-T6 forgings are still below those obtained in the T8 temper, e.g. for 8090-T851 extrusions, tensile properties are 77.6 ksi YS and 84.1 ksi UTS, while for 2091-T851 extrusions, tensile properties are 73.3 ksi YS and 84.1 ksi UTS. By contrast, the alloys of the present invention possess highly improved properties compared to conventional 8090 and 2091 alloys in cold worked tempers, and possess even greater improvements in non-cold worked tempers.
Alloy 2090 comprises 2.4-3.0 Cu, 1.9-2.6 Li and 0-0.25 Mg. The alloy was designed as a low-density replacement for high strength alloys such as 2024 and 7075. However, it has weldment strengths that are lower than those of conventional weldable alloys such as 2219 which possesses typical weld strengths of 35-40 ksi. As cited in the following reference, in the T6 temper alloy 2090 cannot consistently meet the strength, toughness, and stress-corrosion cracking resistance of 7075-T73 (see "First Generation Products--2090," Bretz, ALITHALITE ALLOYS: 1987 UPDATE, J. Kar, S. P. Agrawal, W. E. Quist, editors, Conference Proceedings of International Aluminum-Lithium Symposium, Los Angeles, CA, 25-26 Mar. 1987, pages 1-40). As a consequence, the properties of current Al-Cu-Li alloy 2090 forgings are not sufficiently high to justify their use in place of existing 7xxx forging alloys.
It should be noted that the addition of Mg to the Al-Cu-Li system does not in its own right cause an increase in alloy strength in high strength tempers. For example 8090 (nominal composition Al--1.3 Cu--2.5 Li--0.7 Mg) does not significantly greater strength compared to nominally Mg-free alloy 2090 (nominal composition Al--2.7 Cu--2.2 Li--0.12 Zr) . Furthermore, Mg-free alloy 2020 of nominal composition Al--4.5 Cu--1.1 Li--0.4 Mn--0.2 Cd is even stronger than Mg containing alloy 8090.
European Patent No. 158,571 to Dubost, assigned to Cegedur Societe de Transformation de l'Aluminum Pechiney, relates to Al alloys comprising 2.75-3.5 Cu, 1.9-2.7 Li, 0.1-0.8 Mg, balance Al and grain refiners. The alloys, which are commercially known as CP276, are said to possess high mechanical strength combined with a decrease in density of 6-9 percent compared with conventional 2xxx (Al-Cu) and 7xxx (Al-Zn-Mg) alloys. While Dubost lists high yield strengths of 498-591 MPa (72-85 ksi) for his alloys in the T6 condition, the elongations achieved are relatively low (2.5-5.5 percent).
U.S. Pat. No. 4,752,343 to Dubost et al, assigned to Cegedur Societe de Transformation de l'Aluminum Pechiney, relates to Al alloys comprising 1.5-3.4 Cu, 1.7-2.9 Li, 1.2-2.7 Mg, balance Al and grain refiners. The ratio of Mg to Cu must be between 0.5 and 0.8. The alloys are said to possess mechanical strength and ductility characteristics equivalent to conventional 2xxx to 7xxx alloys. While the purpose of Dubost et al is to produce alloys having mechanical strengths and ductilities comparable to conventional alloys, such as 2024 and 7075, the actual strength/ductility combinations achieved are below those attained by the alloys of the present invention.
U.S. Pat. No. 4,652,314 to Meyer, assigned to Cegedur Societe de Transformation de l'Aluminum Pechiney, is directed to a method of heat treating Al-Cu-Li-Mg alloys. The process is said to impart a high level of ductility and isotropy in the final product. The highest yield strength in the longitudinal direction achieved by Meyer is 504 MPa (73 ksi) for a cold worked, artificially aged alloy, which is significantly below the yield strengths attained in the alloys of the present invention in the cold worked, artificially aged condition.
U.S. Pat. No. 4,526,630 to Field, assigned to Alcan International Ltd., relates to a method of heat treating Al-Li alloys containing Cu and/or Mg. The process, which constitutes a modification of conventional homogenization techniques, involves heating an ingot to a temperature of at least 530.degree. C. and maintaining the temperature until the solid intermetallic phases present within the alloy enter into solid solution. The ingot is then cooled to form a product which is suitable for further thermomechanical treatment, such as rolling, extrusion or forging. The process disclosed is said to eliminate undesirable phases from the ingot, such as the coarse copper-bearing phase present in prior art Al-Li-Cu-Mg alloys.
European Patent Application No. 227,563, to Meyer et al, assigned to Cegedur Societe de Transformation de l'Aluminum Pechiney, relates to a method of heat treating conventional Al-Li alloys to improve exfoliation corrosion resistance while maintaining high mechanical strength. The process involves the steps of homogenization, extrusion, solution heat treatment and cold working of an Al-Li alloy, followed by a final tempering step which is said to impart greater exfoliation corrosion resistance to the alloy, while maintaining high mechanical strength and good resistance to damage. Alloys subjected to the treatment have a sensitivity to the EXCO exfoliation test of less than or equal to EB (corresponding to good behavior in natural atmosphere) and a mechanical strength comparable with 2024 alloys. Meyer et al list broad ranges of alloying elements which, when combined with Al, can produce alloys that may be subjected to the final tempering treatment disclosed. The ranges listed include 1-4 Li, 0-5 Cu, and 0-7 Mg. While the reference lists very broad ranges of alloying elements, the actual alloys which Meyer et al utilize are the conventional alloys 8090, 2091, and CP 276. Thus, Meyer et al do not teach the formation of new alloy compositions, but merely teach a method of processing known Al-Li alloys. The highest yield strength achieved in accordance with the process of Meyer et al is 525 MPa (76 ksi) for alloy CP 276 ) 2.0 Li, 3.2 Cu, 0.3 Mg, 0.11 Zr, 0.04 Fe, 0.04 Si, balance Al) in the cold worked, artificially aged condition. In addition, the final tempering method of Meyer et al is said to improve exfoliation corrosion resistance in Al-Li alloys, whereby sensitivity to the EXCO exfoliation corrosion test is improved to a rating of less than or equal to EB. In contrast, the alloys of the present invention do not require a final tempering step in order to achieve a favorable level of exfoliation corrosion resistance.
U.K. Patent Application No. 2,134,925, assigned to Sumitomo Light Metal Industries Ltd., is directed to Al-Li alloys having high electrical resistivity. The alloys are suitable for use in structural applications, such as linear motor vehicles and nuclear fusion reactors, where large induced electrical currents are developed. The primary function of Li in the alloys of Sumimoto is to increase electrical resistivity. The reference lists broad ranges of alloying elements which, when combined with Al, may produce structural alloys having increased electrical resistivity. The disclosed ranges are 1.0-5.0 Li, one or more grain refiners selected from Ti, Cr, Zr, V and W, and the balance Al. The alloy may further include 0-5.0 Mn and/or 0.05-5.0 Cu and/or 0.05-8.0 Mg. Sumitomo discloses particular Al-Li-Cu and Al-Li-Mg based alloy compositions which are said to possess the improved electrical properties. Sumitomo also discloses one Al-Li-Cu-Mg alloy of the composition 2.7 Li, 2.4 Cu, 2.2 Mg, 0.1 Cr, 0.06 Ti, 0.14 Zr, balance aluminum, which possesses the desired increase in electrical resistivity. The strengths disclosed by Sumitomo are far below those achieved in the present invention. For example, in the Al-Li-Cu based alloys listed, Sumitomo gives tensile strengths of about 17-35 kg/mm.sup.2 (24-50 ksi). In the Al-Li-Mg based alloys listed, Sumitomo discloses tensile strengths of about 43-52 kg/mm.sup.2 (61-74 ksi).
U.S. Pat. No. 3,306,717 to Lindstrand et al relates to filler metal for welding Al-Zn-Mg base alloys. The filler metal comprises Al with 2-8 weight percent Mg, 0.1-10 weight percent Ag, and up to 8 weight percent Si. In addition, the filler metal may contain up to 1.0 weight percent each of Mn and Cr, up to 0.5 weight percent each of Cu, Ti and V, and up to 0.1 weight percent each of Li, Zr and B. The only example given by Lindstrand et al lists a filler metal composition of Al--5 Mg--0.9 Ag.
It should be noted that prior art Al-Cu-Li-Mg alloys have almost invariably limited the amount of Cu to 5 weight percent maximum due to the known detrimental effects of higher Cu content, such as increased density. According to Mondolfo, amounts of Cu above 5 weight percent do not increase strength, tend to decrease fracture toughness, and reduce corrosion resistance (Mondolfo, pp. 706-707.) These effects are thought to arise because in Al-Cu engineering alloys, the practical solid solubility limit of Cu is approximately 5 weight percent, and hence any Cu present above about 5 weight percent forms the less desired primary theta-phase. Moreover, Mondolfo states that in the quaternary system Al-Cu-Li-Mg the Cu solubility is further reduced. He concludes, "The solid solubilities of Cu and Mg are reduced by Li, and the solid solubilities of Cu and Li are reduced by Mg, thus reducing the age hardening and the UTS obtainable." (Mondolfo, p.641). Thus, the additional Cu should not be taken into solid solution during solution heat treatment and cannot enhance precipitation strengthening, and the presence of the insoluble theta-phase lowers toughness and corrosion resistance.
One reference that teaches the use of greater than 5 percent Cu is U.S. Pat. No. 2,915,391 to Criner, assigned to Alcoa. The reference discloses Al-Cu-Mn base alloys containing Li, Mg, and Cd with up to 9 weight percent Cu. Criner teaches that Mn is essential for developing high strength at elevated temperatures and that Cd, in combination with Mg and Li, is essential for strengthening the Al-Cu-Mn system. Criner does not achieve properties comparable to those of the present invention, i.e., ultra high strength, strong natural aging response, high ductility at several technologically useful strength levels, weldability, resistance to stress corrosion cracking, etc.
The following references disclose additional Al, Cu, Li and Mg containing alloys: U.S. Pat. No. 4,603,029 to Quist et al; U.S. Pat. No. 4,661,172 to Skinner et al; European Patent Application Publication No. 0188762 to Hunt et al; European Patent Application Publication No. 0149193; Japanese Patent No. J6-0238439; Japanese Patent No. J6-1133358; and Japanese Patent No. J6-1231145. None of these references disclose the use of silver or zinc as alloying additions.
U.S. Pat. No. 4,584,173 to Gray et al relates to Al-Li-Mg base alloys containing minor amounts of Cu. The alloys comprise 2.1-2.9 percent Li, 3.0-5.5 percent Mg, and 0.2-0.7 percent Cu. In addition, Gray et al disclose that Zn may be added to these alloys in the range of 0-2.0 percent.
U.S. Pat. No. 4,758,286 to Dubost et al relates to aluminum-base alloys comprising 0.2-1.6 percent Cu, 1.8-3.5 percent Li, and 1.4-6.0 percent Mg. Dubost et al teach that up to 0.35 percent Zn may be included in the alloys. However, none of the Al-Cu-Li-Mg alloys actually produced by Dubost et al contain zinc.
U.S. Pat. No. 4,626,409 to Miller discloses aluminum-base alloys comprising 1.6-2.4 percent Cu, 2.3-2.9 percent Li, and 0.5-1.0 percent Mg. Miller teaches that up to 2.0 percent Zn may be added to these alloys, but none of the specific alloys produced by Miller contain zinc.
U.S. Pat. No. 4,648,913 to Hunt et al, assigned to Alcoa, the disclosure of which is hereby incorporated by reference, relates to a method of cold working Al-Li alloys wherein solution heat treated and quenched alloys are subjected to greater than 3 percent stretch at room temperature. The alloy is then artificially aged to produce a final alloy product. The cold work imparted by the process of Hunt et al is said to increase strength while causing little or no decrease in fracture toughness of the alloys. The particular alloys utilized by Hunt et al are chosen such that they are responsive to the cold working and aging treatment disclosed. That is, the alloys must exhibit improved strength with minimal loss in fracture toughness when subjected to the cold working treatment recited (greater than 3 percent stretch) in contrast to the result obtained with the same alloy if cold worked less than 3 percent. Hunt et al broadly recite ranges of alloying elements which, when combined with Al, may produce alloys that are responsive to greater than 3 percent stretch. The disclosed ranges are 0.5-4.0 Li, 0-5.0 Mg, up to 5.0 Cu, 0-1.0 Zr, 0-2.0 Mn, 0-7.0 Zn, balance Al. While Hunt et al disclose very broad ranges of several alloying elements, there is only a limited range of alloy compositions that would actually exhibit the required combination of improved strength and retained fracture toughness when subjected to greater than 3 percent stretch. In contrast, large amounts of stretch are not required in order to produce favorable properties in the alloys of the present invention. In addition, the yield strengths attained in the alloys of the present invention are substantially above those achieved in the alloy compositions of Hunt et al. Further, Hunt et al indicate that it is preferred in their process to artificially age the alloy after cold working, rather than to naturally age. In contrast, the alloys of the present invention exhibit an extremely strong natural aging response.
U.S. Pat. No. 4,795,502 to Cho, assigned to Alcoa, the disclosure of which is hereby incorporated by reference, is directed to a method of producing unrecrystallized wrought Al-Li sheet products having improved levels of strength and fracture toughness. In the process of Cho, a homogenized aluminum alloy ingot is hot rolled at least once, cold rolled, and subjected to a controlled reheat treatment. The reheated product is then solution heat treated, quenched, cold worked to induce the equivalent of greater than 3 percent stretch, and artificially aged to provide a substantially unrecrystallized sheet product having improved levels of strength and fracture toughness. The final product is characterized by a highly worked microstructure which lacks well-developed grains. The Cho reference appears to be a modification of the Hunt et al reference listed above, in that a controlled reheat treatment is added prior to solution heat treatment which prevents recrystallization in the final product formed. Cho broadly states that aluminum base alloys within the following compositional ranges are suitable for the recited process: 0.5-4.0 Li, 0-5.0 Mg, up to 5.0 Cu, 0-1.0 Zr, 0 -2.0 Mn, and 0-7.0 Zn. As in the Hunt et al reference, the particular alloys utilized by Cho are apparently chosen such that they exhibit a combination of improved strength and fracture toughness when subjected to greater than 3 percent cold work. The alloys of Cho must further be susceptible to the reheat treatment recited. While Cho provides a process which is said to increase strength in known Al-Li alloys, such as 2091, the strengths attained are substantially below those achieved in the alloys of the present invention. Cho also indicates that artificial aging should be used in his process to obtain advantageous properties. In contrast, the alloys of the present invention do not require artificial aging. Rather, the present alloys exhibit an extremely strong natural aging response which permits their use in applications where artificial aging is impractical.
U S. Patent Application Serial No. 07/327,666 of Pickens et al, filed Mar. 23, 1989, which is a Continuation-In-Part of U.S. patent application Ser. No. 07/233,705 filed Aug. 18, 1988, and which is hereby incorporated by reference, discloses Al-Cu-Mg-Li alloys having compositions within the following ranges: 5-7 Cu, 0.1-2.5 Li, 0.05-4 Mg, 0.01-1.5 grain refiner selected from Zr, Cr, Mn, Ti, Hf, V, Nb, B and TiB.sub.2, and the balance aluminum. U.S. patent application Ser. No. 07/327,666 also discloses Al-Cu-Mg-Li alloys of lower Cu content having compositions within the following ranges: 3.5-5 Cu, 0.8-1.8 Li, 0.25-1.0 Mg, 0.01-1.5 grain refiner selected from Zr, Cr, Mn, Ti, Hf, V, Nb, B and TiB.sub.2, and the balance aluminum. The Ser. No. 07/327,666 application further teaches that ancillary elements such as Ge, Sn, Cd, In, Be, Sr, Ca and Zn may be added, singly or in combination, in amounts of from about 0.01 to about 1.5 weight percent.
U.S. patent application Ser. No. 07/327,927, of Pickens et al, filed Mar. 23, 1989, which is a continuation-in-part of U.S. patent application Ser. No. 07/083,333, filed Aug. 10, 1987, and which is hereby incorporated by reference, discloses an Al-Cu-Mg-Li-Ag alloy with compositions in the following broad range: 0-9.79 Cu, 0.05-4.1 Li, 0.01-9.8 Mg, 0.01-2.0 Ag, 0.05-1.0 grain refiner selected from Zr, Cr, Mn, Ti, B, V, Hf and TiB.sub.2, and the balance Al. No disclosure is made in the Ser. No. 07/327,927 application of the addition of zinc, or any of the elements germanium, tin, cadmium, indium, beryllium, strontium, scandium, yttrium and calcium, to the Al-Cu-Mg-Li-Ag alloys. In accordance with the present invention, Zn additions may be used to reduce the levels of Ag present in the alloys taught in the Ser. No. 07/327,927 application. Thus, Zn may be substituted for a portion of the Ag, thereby reducing costs. Tensile properties are improved in the Zn containing alloys of the present invention. Additionally, resistance to stress corrosion cracking may also be improved.